To be useful as supplements for feeding cattle and swine, calcium phosphate products must contain very low proportions of fluorine, (see, for example, U.S. Pat. No. 4,243,643 to Mills). When such low fluorine content products are made from phosphate rock, a typical process involves making a reaction mixture of phosphate rock, a sodium compound and phosphoric acid, which reaction mixture is then calcined at high temperatures (1900.degree. F. or higher) to cause fluorine to evolve. At such high temperatures detrimental melting can occur if the impurity content of the reaction mixture (especially SiO.sub.2) is too high for the particular calcination temperature required to drive off the desired amount of fluorine. Accordingly, it has heretofore been necessary to use only the higher quality phosphate rocks in making such defluorinated products by calcination. The present invention provides a means of using lower grade phosphate rock (e.g., less than about 72% BPL) in such calcination mixtures and means of minimizing the loss of P and Na during the calcination.
The invention involves controlling the amount of phosphoric acid in the mixture such that the ratio of phosphorus in percentage by weight on a dry basis in said mixer-to-the phosphorus desired in percentage by weight in the calcined product is substantially equivalent to the ratio of the calcium in percentage by weight on a dry basis in the mixture-to-the calcium in percentage by weight in the calcined product.
It is also useful, in one non-limiting embodiment of the invention, to control the weight ratio of P in the phosphoric acid to P in the rock so as to prevent substantial fusion.
U.S. Pat. Nos. 4,243,643 and 2,995,439 are illustrative of the processes for making such defluorinated phosphate rock products using sodium carbonate. In the latter patent, it is noted that soda ash containing over 98% Na.sub.2 CO.sub.3 is preferably prereacted with phosphoric acid and the reagent mixture mixed with the phosphate rock, then introduced into the calcination apparatus. This patent further prefers to premix the reagents and the phosphate rock, (advantageously in a pug mill or the like), and further notes that sodium compounds other than soda ash, such as sodium hydroxide, can be used in the reaction mixture. This patent does not specifically describe how to react low BPL phosphate rock, merchant grade phosphoric acid, caustic and water in order to produce a feed suitable for calcination to make such a low fluorine containing product without detrimental melting, in the manner of the present invention.
U.S. Pat. No. 2,997,367, to Williams, in an example, shows contacting phosphate rock with a merchant grade strength acid (54% P.sub.2 O.sub.5) and soda ash. No water is added in this example nor is caustic soda mentioned. This patent provides a correlation between calcium, P.sub.2 O.sub.5, SO.sub.3, F, Mg, Na, K.sub.2 O, Fe.sub.2 O.sub.3 and Al.sub.2 O.sub.3 in the feed. It shows no correlation, such as used in the present invention, between the P.sub.2 O.sub.5 content of the phosphoric acid being used and the P.sub.2 O.sub.5 content of the phosphate in the rock, which could be used to permit the use of low BPL phosphate rock in such a product with a minimum loss of added phosphoric acid and/or sodium during the calcination and without detrimental melting.
British patent specification No. 902,361, published Aug. 1, 1962 of Hollingsworth, provides a correlation between CaO, Na.sub.2 O, P.sub.2 O.sub.5 and SiO.sub.2. At page 2, column 1 lines 32-57 there is a discussion of the practice in the United States at that time of maintaining the amount of added phosphoric acid at about 3.6% (calculated as P.sub.2 O.sub.5). However, Hollingsworth teaches against this practice and urges the use of between 7 and 12% added P.sub.2 O.sub.5 (see page 2 column 2 lines 66-106). This patent does not provide a means of utilizing low BPL rock by means of the correlation and other improvements of the present invention.
Other related U.S. Pat. Nos. of Hollingsworth and of Hollingsworth with other inventors are 2,478,200; 2,479,389; 2,531,046; 2,556,541; 2,556,542; 2,753,253; 2,754,191; 2,778,722; 3,151,936; 3,151,941, and, 3,364,008.
A history of the development of such defluorinated phosphate rock products, both for use as a fertilizer and as a mineral supplement for animal feeds, is described in volume 41 Industrial and Engineering Chemistry, No. 7 pages 1325-1327, by Whitney and Hollingsworth.
U.S. Pat. No. 3,002,812 to Williams notes that the phosphorus pentoxide content in a saleable ore varies from 30% to 36%, corresponding to a BPL of about 65% to about 75%, i.e., (% P.sub.2 O.sub.5 multiplied by 310/142).
U.S. Pat. No. 3,058,804 to Tynan has examples wherein defluorinated products are made from Florida phosphate rock of approximately 77% BPL by blending the rock with phosphoric acid for about 3 minutes in a pug mill and then adding sodium carbonate and pug milling for 5 additional minutes. In this patent the phosphoric acid is described as being made from 72 BPL rock, and no mention is made of using such a low BPL rock other than as a source for phosphoric acid manufacture.
U.S. Pat. Nos. 3,099,530; 3,142,534; and 3,151,937 to Nickerson show various processes in which steam is used to assist the defluorination of calcine reaction mixtures.
Also relevant are U.S. Pat. Nos. 3,058,804 to Tynan and 3,292,995 to Allen, Australian Pat. No. 242,653 published Dec. 13, 1962 and Canadian Pat. No. 689,205 issued June 23, 1964, both to Hollingsworth.
It can be seen from the above discussion of the prior art, that there is a need for a process which (a) produces a reaction mixture, from phosphate rock, phosphoric acid, caustic and water and low BPL rock (e.g., less than about 73% BPL), and (b) converts the reaction mixture by calcination, without detrimental melting, to a low fluorine containing product suitable for use as a fertilizer or, more preferred, as an animal feed supplement.
All percentages herein are by weight unless otherwise specified.
In this application, a reference to calcium is intended to also include calcium oxide as an equivalent. Calcium is referred to primarily because the usual laboratory analysis is for calcium, which is then sometimes reported as CaO. Furthermore, any other inert element or compound in the phosphoric rock which is not volatile at the calcination temperature, (e.g., Fe, Al, Mg) can be used to provide the same ratio, and is an equivalent to calcium in that respect in this invention.